The present invention relates to a method in the measurement and control of the short circulation and the headbox of a paper machine or equivalent. The present invention relates also to a headbox and the short circulation.
The invention relates to the arrangement of the short circulation and the headbox of a paper machine or equivalent. The paper machine or equivalent refers in the present context to a machine with which paper-like product such as paper, board or tissue paper is produced.
The stock feeding of the paper machine is in general as follows. The stock components are stored in the paper mill in separate storage tanks, wherefrom they are fed into proportioning tanks and therefrom further into a common mixing tank, in which the stock components are intermixed. From the mixing tank the stock is fed into a machine tank, wherefrom the stock, being in general in about 3% consistency, is fed into a short-circulation wire pit. In the wire pit the thick stock is diluted into a headbox consistency, which is in general about 1%.
The fibres and fillers to be used as raw material are taken to the wire through a headbox and conveyed by water. The filtrate having passed through the wire, containing fibrous agents and fillers in great quantities, is returned as a filtrate of the thick stock from the machine tank back to the wire through the headbox. A flow link thus formed is called a short circulation.
Impurities may enter in the short circulation together with the thick stock or through other ways which have to be removed before the headbox. This is carried out with short-circulation cleaning apparatus, such as hydrocyclones, screens, machine screens and deaeration tanks.
The short circulation together with the headbox in connection therewith is in general considered as the most sensitive part of the papermaking process. Any small changes in the consistency, flow or other parameters immediately affect the quality of the paper being manufactured or cause web breaks on the paper machine. The function of the short circulation in the papermaking is, among other things, to produce a fibre suspension of uniform quality, in which the various components (fibre fractions, chemicals and fillers) are intermixed into a homogeneous fluid. The good homogeneity of the stock thus produced will guarantee a uniform quality in the paper and an undisturbed production process in subsequent phases.
In the papermaking, one of the important functions of the headbox is formation of slice jet to be optimal in its flow state. In an optimal slice jet, the solid matter is distributed homogeneously, the floc size is optimal, the disturbances are minimal, and the turbulence level is under control. The essential measurable and controllable quantity is the degree of suspension fluidization, illustrating the intermobility of fibres. In the headbox, various geometrical designs are used for fluidizing the suspension, such as step changes of flow channels, adjustments of trailing elements and various surface phenomena, such as boundary layer turbulence, wherewith turbulence is generated in the flow of the suspension.
In the prior-art OptiFeed process of Metso Paper, Inc., described in patent specification FI-103676, the stock entering the paper machine is built from a number of separate (2-4) stock components, the fibrous properties of which deviate from each other and vary along with changes of the paper grade to be manufactured. The stock components are mixed into homogeneous fluid in so-called mixing reactors located in the parts of the short circulation to which several stock components are brought simultaneously.
The operation of the OptiFeed process is dependent on the goodness of the operation of the mixing reactor. In an optimal situation, the mixing should be as perfect as possible and in addition, to work for all paper grades being manufactured, even though the flow quantities of different components, depending on the quality, may vary to a great extent. The goodness of the mixing of different components being mixed as known in the art can be measured and controlled when an optimal operation is to be secured. The mixability of suspensions containing fibrous matter is dependent on the mobility of solid matter and the turbulence generated therethrough. Optimizing the generation of turbulence is implementable e.g. by means of various adjustable throttling elements, disclosed e.g. in the patent application of Metso Paper, Inc. No. FI-992015.
In the flow of fibre suspension, the fibres tend to form accumulations called flocs. If the consistency of the flow exceeds the sedimentation consistency, the flocs are built into a net-like united phase, which in the papermaking is an undesired state. The state in which the structure is completely decomposed is called fluidized. In mixing a fibrous suspension, momentary fluidization of components to be mixed is expected. The mixing is in general carried out by conducting flows of different components into one at different speeds. If the state of fluidization can be monitored, the differential speed required (shearing stress between the flows being mixed) can be set optimal. In connection with the fluidization, the structure of a fibre net or flocs is decomposed. Hereby, the state of fluidization can be estimated with the aid of floc size and its completeness with the aid of the minimum size achieved. The floc size measuring in the process circumstances is very difficult in practice.
Generating turbulence in the fibre suspension causes breaking up of fibre flocs and increased intermobility of individual fibres. Providing fluidization by increasing the turbulence requires geometric changes to add shearing stresses or a surface of a flow channel or a trailing element to produce sufficient boundary-layer turbulence. Increased fluidization as such will not cause reduction of turbulence. The fibre suspension usually tends to become re-flocculated so to speak, which can be observed as reduced fibre mobility (degree of fluidization). On the other hand, the properties of turbulence include so-called dissipation, which means changing of the kinetic energy of the turbulence into internal energy (heat) of the fluid. However, the degree of fluidization of the suspension will be decreased owing to the dissipation of turbulence. Thus, the fluidization of the suspension is a transient state, the follow-up of which is essential for the success of the papermaking process.
The objective of the present invention is to develop a method and an apparatus for real-time measuring of the short circulation of a paper machine or equivalent and of the fibre mobility of the suspension of the headbox and for controlling the flow state.
The objective of the present invention is also to provide a method and an apparatus, wherewith as optimal mixability of the fibre suspension as possible is guaranteed in different parts of the short circulation and the headbox so that the fibre suspension is in an optimal state for the subsequent phase of the process.
The method according to the invention is mainly characterized in that in the method, the selected measurement targets are provided with means for measuring the fibre mobility of the fibre suspension, and on the basis of the fibre mobility measured from the fibre suspension, the flow state of the fibre suspension is controlled.
The headbox of the invention is in turn characterized in that the headbox comprises sensors on the width of the headbox or a traversing sensor/sensors, being fitted in different width points of the headbox, and that a sensor/sensors is/are arranged to measure the fibre mobility profile of the headbox on the width of the entire headbox and that the headbox comprises means for changing the flow state on the basis of the measurement data obtained from the sensors.
The short circulation of the invention is characterized in that the short circulation comprises a sensor/sensors, disposed in the pipes of the short circulation and/or in the cleaning apparatus, and that a sensor/sensors are arranged to measure the fibre mobility of the fibre suspension in the short circulation and that the short circulation comprises means for changing the flow state on the basis of the measurement data rendered by the sensors.
According to the invention, the apparatus components used for controlling the mixture in the short circulation of a paper machine or equivalent are provided with sensors measuring the flow state, on the basis of the data obtained wherefrom the flow state is controlled by means of control devices. With a measurement and control system such as this, the furnish of the fibre suspension is controlled to be such that it is optimal for the next process phase. The solution according to the invention can be used also in cardboard and tissue machines.
In addition, with the method according to the invention, the fibre mobility of the suspension of the headbox can be measured most precisely on the entire width of the slice channel. The slice channel is provided with a row of sensors, a sensor matrix or a traversing sensor, in which the degree of fluidization of the suspension is measured in real time. In multiple-layer headboxes, a row of sensors or a sensor matrix is positioned on each layer. On the basis of the measurement data obtained from the sensors, the flow state of the suspension of the headbox is controlled, in order to make the fibre mobility, that is, the degree of fluidization, optimal. When the optimal range of variation of the fibre mobility is known for different paper grades, the quality of the paper produced can be controlled in changing running circumstances. The optimal range of variation of the fibre mobility can be determined experimentally.